Small Packaged Tunable Laser with Beam Splitter

ABSTRACT

According to one embodiment, the present application includes a tunable laser configured in a small package. The tunable laser includes a housing with a volume formed by exterior walls. An electrical input interface is positioned at the first end of the housing and configured to receive an information-containing electrical signal. An optical output interface is positioned at the second end of the housing and configured to transmit a continuous wave optical beam. A tunable semiconductor laser is positioned in the interior space and operable to emit a laser beam having a selectable wavelength. A focusing lens assembly is positioned in the interior space along an optical path of the laser beam to operatively couple the laser beam to the optical output interface.

REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional application No. 61/444,362, filed Feb. 18, 2011, and herein incorporated by reference in its entirety. The present application is also a continuation-in-part of U.S. patent application Ser. No. 12/722,825, filed Mar. 12, 2010, which in turn is a continuation-in-part of U.S. patent application Ser. No. 12/537,026, filed Aug. 6, 2009, each of which are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present application is directed to a tunable laser and, more particularly, to a small, packaged tunable laser.

BACKGROUND

Tunable lasers may be packaged as a component of an optical transceiver, or may be used in other applications outside of an optical transceiver. Tunable lasers are generally packaged with other components including an electrical interface and an optical interface.

There is an ever-constant challenge in the industry to reduce the size of tunable laser packages. The reduction in size may allow lasers to be used in a greater number of applications. The reduction in size provides numerous design challenges for the package components to fit within the limited space and also not compromise performance or reliability.

In applications in which tunable lasers are a component of an optical transceiver, the tunable lasers should be sized for use with one of the various form factors. The various form factors provide standardized dimensions and electrical input/output interfaces that allow devices from different manufacturers to be used interchangeably. Examples of form factors include but are not limited to XENPAK, SFF (“Small Form Factor”), SFP (“Small Form Factor Pluggable”), and XFP (“10 Gigabit Small Form Factor Pluggable”).

Therefore, there is a need for a small, packaged tunable laser for various applications.

SUMMARY

The present application is directed to tunable lasers configured in a small package. The tunable lasers may include a rectangular housing, an electrical input interface, an optical output interface, a tunable semiconductor laser and a focusing lens assembly. The rectangular housing has a volume of less than 0.6 cubic centimeters, with six planar exterior walls including a bottom, a top, opposing first and second ends, and opposing sidewalls. The exterior walls form a hermetically sealed interior space that includes a major axis that extends through the first and second ends. The electrical input interface is positioned at the first end of the housing and aligned with the major axis. The electrical interface is configured to receive an information-containing electrical signal. The optical output interface is positioned at the second end of the housing and aligned with the major axis. The optical interface is configured to transmit a continuous wave optical beam. The tunable semiconductor laser is positioned in the interior space and operable to emit a laser beam having a selectable wavelength. The focusing lens assembly is positioned in the interior space along an optical path of the laser beam to operatively couple the laser beam to the optical output interface.

The present invention is not limited to the above features and advantages. Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective view of a small, packaged tunable laser according to one embodiment; and

FIG. 2 is a side elevational view of the interior components in the tunable laser of FIG. 1.

DETAILED DESCRIPTION

The present application is directed to a small, packaged tunable laser 100 as illustrated in FIG. 1. The tunable laser 100 is packaged in a housing 200 that forms an interior space for housing the laser components 300. The laser 100 is packaged in housing which is particularly small and compact in size which makes it especially suitable for use in pluggable optical transceivers and various other module configurations or applications.

The housing 200 includes a generally rectangular body 206 with exterior walls that forms a substantially rectangular shape. The body 206 includes a bottom 204, a cover (not illustrated), first and second ends 230, 231, and opposing sidewalls 232, 233. The cover may be substantially planar and positioned on the top surfaces of the first and second ends 230, 231 and opposing sidewalls 232, 233. In one embodiment, the cover is substantially identical to the bottom 204.

The housing 200 includes a substantially rectangular shape with a width W formed by the opposing sidewalls 232, 233, a length L formed by the first and second ends 230, 231, and a height H that extends between the bottom 204 and top of the sidewalls 232, 233 and ends 230, 231. The housing 200 may include various sizes. In one specific embodiment, the width W is about 5.4 mm, the length L is about 17.1 mm, and the height H is about 5.9 mm. The volume of the interior space formed by the housing 200 may also vary depending upon the application. Exemplary volumes may range from between about 400 mm³ to about 600 mm³. In one specific embodiment, the volume is about 545 mm³. The housing 200 includes an elongated shape with a major axis X extending along the length L through the first and second ends 230, 231, and a minor axis Y perpendicular to the major axis and extending through the opposing sidewalls 232, 233. The housing 200 may be hermetically sealed to protect the laser components 300 from humidity and other environmental conditions.

A flexible electrical cable interface 202 extends outward from the first end 230 of the housing 200. The electrical interface 202 is configured to receive power and information-containing electrical signals. In the embodiment of FIG. 1, the electrical interface 202 includes a connector portion 201 that is aligned with the major axis X, and includes various electrical connection pads 203. The electrical interface 202 may also include additional flexible cables that extend outward from the first end 230, or sidewalls 232, 233.

An optical output interface 800 extends outward from the second end 231 of the housing 200. In one embodiment, the optical output interface 800 is an optical fiber ferrule or “pigtail” aligned with the major axis X of the housing 200. The optical output interface 800 is configured to transmit a continuous wave optical beam that is emitted from the laser components 300 over an optical fiber contained within the interface.

The laser components 300 generally include an external cavity laser and coupling optics 320. The external cavity laser 310 includes a first subassembly including a diode gain chip 311 comprising a Fabry-Perot diode laser with a substantially non-reflective front facet and a highly reflective rear facet. The gain chip 311 may also include a bent-waveguide structure. The first subassembly also includes a collimating lens 314 and a steering lens 315 in the path of the beam emitted from the gain chip 311. The collimating lens 314 and the steering lens 315 are both mounted on the same base as the gain chip 311.

The external cavity laser 310 further includes a second subassembly including a tunable filter 316 (including tunable filter elements 316 a and 316 b), a cavity length actuator 317 disposed between the tunable filter elements 316 a and 316 b, and a reflective element 319.

Possible implementations of the tunable filter 316 include but are not limited to Bragg gratings, Fabry-Perot etalons, and liquid crystal waveguides. In one embodiment, the tunable filter 316 includes a pair of spaced apart tunable elements or etalons 316 a, 316 b. The etalons 316 a, 316 b are Fabry-Perot spaced etalons that are positioned in a parallel configuration. The first etalon 316 a includes a thickness measured between opposing faces and a refractive index according to the material from which it is constructed. The second etalon 316 b includes a thickness measured between its opposing faces and a refractive index according to the material from which it is constructed. The etalons 316 a, 316 b may be constructed from the same or different materials, and may include the same or different thicknesses. Etalons 316 a, 316 b may be constructed from various materials, such as but not limited to silicon and gallium arsenide. One or both etalons 316 a, 316 b are tunable by a temperature-induced change in their refractive indexes and/or a temperature-induced change in their thickness. In one embodiment, the etalons 316 a, 316 b are tunable by simultaneous control of both the refractive index and the physical thickness.

In one embodiment, the tunable filter 316 utilizes a Vernier tuning mechanism including respective first and second optical filters 316 a, 316 b having respective sets of transmission peaks having slightly different free spectral ranges and similar finesses. Tuning is performed by shifting the set of transmission peaks of the second optical filter 316 b relative to the set of transmission peaks of first optical filter 316 a to align a single transmission peak of each of the first and second sets of transmission peaks.

The actuator 317 may be a block of silicon which is polished and anti-reflection coated, and is disposed between etalons 316 a and 316 b. In some embodiments the actuator 317 may use thermal, mechanical, or electro-optical mechanisms to adjust the optical pathlength of the laser cavity. In some embodiments the actuator 317 is oriented in the cavity with a seven degree tilt with respect to the optical axis of the beam directed to it from etalon 316 a, so that the light circulating in the optical cavity does not reflect back into the cavity regardless of the efficacy of the anti-reflection coating. The actuator 317 may also lock the optical path length.

The external cavity tunable laser 310 may be configured with the tunable filter 316 a/316 b being decoupled from the gain chip 311. This configuration results in the tunable filter 316 a/316 b being very stable and therefore does not require an external wavelength locker as required in Distributed Feedback (DFB) lasers and Distributed Bragg Reflector (DBR) lasers. Other advantages of the external cavity tunable laser 310 over these other lasers are the extremely narrow linewidth and very high side mode suppression ratio.

The coupling optics 320 provide isolation and coupling to the optical output interface 801. The coupling optics 320 efficiently couple light from the gain chip 311 to the optical output interface 801. A total optical magnification of the coupling optics 320 (including . . . ) and the external cavity lenses 314, 315 is chosen to correct for the difference between mode field diameters of the gain chip 311 and the optical fiber 802. The coupling optics 320 includes an optical isolator 324. The optical isolator 324 may include a two-stage isolator that prevents light reflected from coupling optics 320 from getting back into the external cavity tunable laser 310. The isolator 324 may also rotate a light polarization by 90 degrees to improve transmission. In one embodiment, the optical path is aligned substantially along the major axis X of the housing 200.

The lens assembly is disposed between the optical isolator 324 of the coupling optics 320 and the optical output interface 800 and includes a wedge shaped support 400 which supports a planar beam splitter 401. The beam splitter 401 is arranged on the support 400 so that its plane is at a 45 degree angle with respect to the incoming beam. A photodiode 402 is mounted on the surface of the base below the beam splitter 401. The beam splitter 401 directs a small portion (e.g. 5%) of the isolator output beam to the photodiode 402, which functions to sense the intensity level of the tunable laser output, so that the user may appropriately control the current to the laser to achieve the desired optical output level or intensity. The remainder of the isolator output beam is directed by a beam splitter to the collimating lens 501.

Downstream from the collimating lens 501 is a window 600 which is attached to the housing 200 and permits the collimated beam to exit the housing 200. Outside of the housing 200 and coupled to the collimated beam is the optical output interface, which includes a fiber focus alignment lens 700 and the optical fiber ferrule 800. In the embodiment depicted, the cladded optical fiber 802 is a “pig-tail” that extends for a relatively short distance to allow coupling to other subassemblies. In another embodiment (not depicted), the window 600 or the alignment lens 700 may be the final downstream components associated with the housing 200, to allow the user to couple an optical fiber directly to the outside of the housing 200 through a pluggable connector or other optical interface.

A thermoelectric cooler includes first and second plates 702 and 704, respectively, separated by intermediate members 703 a, 703 b, 703 c, etc., that provides a base for supporting the various elements of the tunable laser 100. In one embodiment, the thermoelectric cooler (comprised of elements 702, 703, 704) is positioned between the bottom 204 of the housing 200 and one or more of the laser components 300. The plates 702, 704 may be constructed from a variety of materials, including ceramics. The intermediate members 703 a, 703 b, etc., each include a first end operatively connected to the first plate 702 and a second end operatively connected to the second plate 704. The intermediate members 703 a, 703 b, etc. are electrically connected in series by connectors. The intermediate members 703 a, 703 b, etc., are constructed from semiconductor material that allows for electron flow through the member 703 a, 703 b, etc. when connected to a DC power source. In use, as the DC power source is activated and a current passes through the series of intermediate members 703 a, 703 b, etc., the current causes a decrease in temperature at the first plate 702 that absorbs heat from the laser components 300. The heat is transferred through the plate 702 and intermediate members 703 a, 703 b, etc., into the second plate 704. This heat may then be transferred from the second plate 704, such as to a heat sink.

Likewise, the temperature of the tunable filter 316 and cavity length actuator 317 may be separately controlled from the other laser components 300. A bench 318, which may be composed of a glass, may provide thermal isolation from the thermoelectric cooler 400.

One example of a tunable laser is disclosed in U.S. Pat. No. 7,257,142, herein incorporated by reference. Such patent describes what may be called an “integrated design” in contrast with an arrangement of discrete components such as described in the present disclosure.

Spatially relative terms such as “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various elements, regions, sections, etc. and are also not intended to be limiting. Like terms refer to like elements throughout the description.

As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.

The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein. 

1. A small, packaged tunable laser comprising: a rectangular housing having a volume of less than 0.6 cubic centimeters, with six planar exterior walls including a bottom, a top, opposing first and second ends, and opposing sidewalls, the exterior walls forming a hermetically sealed interior space that includes a major axis that extends through the first and second ends; an electrical input interface positioned on the exterior of the housing and configured to receive an information-containing electrical signal; an optical output interface positioned on the exterior of the housing and aligned with the major axis, the optical output interface configured to transmit a continuous wave optical beam; a tunable semiconductor laser positioned in the interior space and operable to emit a laser beam having a selectable wavelength; a beam splitter positioned in the interior space in the path of the laser beam for producing a first beam directed to the optical output interface, and a second beam; and a photodiode disposed in the path of the second beam for determining the emitted intensity of the laser beam.
 2. The tunable laser of claim 1, wherein the tunable semiconductor laser is an external cavity laser that includes a tunable filter.
 3. The tunable laser of claim 2, wherein the tunable filter comprises a Vernier tuning mechanism including respective first and second optical filters having respective sets of transmission peaks having slightly different free spectral ranges and similar finesses, and wherein tuning is performed by shifting the set of transmission peaks of the second optical filter relative to the set of transmission peaks of the first optical filter to align a single transmission peak of each of the first and second sets of transmission peaks.
 4. The tunable laser of claim 1, further comprising a base in the interior of the housing; a structure operatively coupled to the base, having a front facet and a substantially non-reflective rear facet optically coupled via a waveguide passing therethrough, the structure further including: a gain section to emit a plurality of photons in response to a first electrical input, having a facet defining the rear facet of the structure.
 5. The tunable laser of claim 1, wherein the optical path is aligned along the major axis of the housing.
 6. The tunable laser of claim 1, further comprising coupling optics positioned in the interior space along the optical path between the semiconductor laser and the optical output interface, the coupling optics including a pair of coupling lenses and an isolator.
 7. The tunable laser of claim 6, further comprising a photodiode disposed between the coupling optics and the optical output interface.
 8. The tunable laser of claim 1, further comprising a focusing lens assembly positioned in the interior space along an optical path of the first beam emitted by the beam splitter to operatively couple the first beam to the optical output interface.
 9. The tunable laser of claim 3, further including a cavity length actuator positioned in the interior space along an optical path of the beam downstream from the first optical filter and functioning to adjust and lock an optical pathlength of the external cavity tunable laser.
 10. The tunable laser of claim 9, wherein the cavity length actuator is a block of silicon having a planar surface tilted with respect to the optical axis to prevent the incoming light from the laser from reflecting back into the cavity of the laser.
 11. The tunable laser of claim 3, further comprising a cavity length actuator disposed between the first and second optical filters of the Vernier tuning mechanism, and further including an anti-reflection coating on its planar surface.
 12. The tunable laser of claim 4, further including a thermoelectric cooler positioned within the interior space between the bottom of the housing and at least one of the tunable semiconductor laser and the coupling optics.
 13. The tunable laser of claim 4, wherein the electrical input interface includes a flexible cable having one end mounted in the interior of the housing and a second end that extends outward from the housing including a conductive connector pad pattern on the cable to enable an electrical connector to be coupled thereto.
 14. A small, packaged tunable laser comprising: a rectangular housing with six planar sides including a bottom, a top, first end, second end, and two opposing sidewalls, the housing including a hermetically sealed interior space with a length measured between the first and second ends and a width measured between the opposing sidewalls, the length being larger than the width; laser components positioned in the interior space and including coupling optics and an external cavity laser with a tunable filter, the laser components aligned within the interior space with an optical path of a laser beam that emanates at the external cavity laser and extends along the coupling optics substantially perpendicular to the first and second ends and along a portion of the length of the housing; an electrical input interface positioned at the first end of the housing and configured to receive an information-containing electrical signal; and an optical output interface positioned at the second end of the housing and configured to transmit a continuous wave optical signal.
 15. The tunable laser of claim 14, further including a cavity length actuator positioned in the interior space along an optical path of the beam from the first optical filter to adjust and lock an optical path length of the external cavity tunable laser.
 16. The tunable laser of claim 15, wherein the cavity length actuator is a block of silicon having a planar surface tilted with respect to the optical axis to prevent the incoming light from the laser from reflecting back into the cavity of the laser.
 17. The tunable laser of claim 3, further comprising a cavity length actuator disposed between the first and second optical filters of the Vernier tuning mechanism, and further including an anti-reflection coating on its planar surface.
 18. The tunable laser of claim 11, wherein the external cavity laser further includes a cavity length actuator to adjust an optical pathlength of the external cavity tunable laser.
 19. The tunable laser of claim 11, wherein the optical interface is a ferrule connected to an optical fiber pigtail.
 20. The tunable laser of claim 14, wherein the coupling optics are positioned in the interior space along the optical path between the external cavity laser and the focusing lens assembly, the coupling optics including a pair of coupling lenses and an isolator.
 21. The tunable laser of claim 15, further comprising a photodiode disposed between the coupling optics and the optical output interface.
 22. The tunable laser of claim 14, wherein the housing has a volume of about 0.55 cubic centimeters. 